In Donetsk, Luhansk and Kharkiv they clashed with police, hung Russian flags from the buildings and called for a referendum on independence.

Ukraine's Acting President called an emergency security meeting in response.

Earlier, Russian President Vladimir Putin has said Moscow has the right to protect the Russian-speaking population there. Ukraine's leaders, in turn, deny the country's Russian speakers are under threat and have said they will resist any intervention in their country.

Ukrainian Acting President Olexander Turchynov canceled a planned visit to Lithuania and called a meeting of the country's security chiefs to deal with the unrest.

In Donetsk, in what was reportedly the day's most violent protest, a large group of activists broke away from a crowd rallying in the main city square to attack and occupy the regional government seat.

After clashing with riot police and breaking through their lines to enter the building, they raised the Russian flag and hung a banner from the building. Protesters outside cheered and chanted: “Russia, Russia.”

Ihor Dyomin, a spokesman for Donetsk local police, said about 1,000 people had taken part in the storming of the building. “Around 100 people are now inside the building and are barricading the building,” he added.

In Luhansk, police fired tear gas at dozens of protesters who broke into the local security service building in an attempt to force the release of 15 pro-Russian activists who were arrested earlier in the week and accused of plotting violent unrest.

And in Kharkiv, several dozen people also entered the regional government building after breaking through police lines.